The present invention relates to a torque split control system for controlling a driving force distribution between front and rear wheels of a four wheel drive vehicle, and more specifically to a torque split control system which can control the driving force distribution adequately in accordance with a wheel speed difference between front and rear wheels without receiving undesired influences of a tire diameter difference between the front and rear wheels.
A conventional driving force distribution control system controls a driving force distribution between front and rear wheels by controlling an engagement force of a torque distributing transfer clutch in accordance with a wheel speed difference between front and rear wheel speeds detected by wheel speed sensors.
However, this conventional control system tends to increases the clutch engagement force excessively especially in a high vehicle speed range when the front and rear wheel tire diameters become different from each other. As shown in FIG. 7, the measured wheel speed difference DVw between the front and rear wheels is equal to a sum of a wheel speed difference DVs due to drive wheel slip, and a wheel speed difference DVt due to a difference between the front wheel tire diameter and the rear wheel tire diameter, and this conventional control system increases the clutch engagement force as shown by a line Tm' in FIG. 7, in accordance with the measured wheel speed difference DVw, without considering the wheel speed difference DVt due to the tire diameter difference. As a result, there arise undesired up-and-down vibrations and an excessive increase in differential oil temperature due to excessive heat generated by clutch slippage (=DVs+DVt).
The tire diameters of the front and rear wheels are more or less unequal because of difference in operating condition such as tire air pressure and wheel load, inequality in tire wear, intentional installation of tires of different sizes, or usage of a temporary spare tire.